Conventionally, a semiconductor device using diamond as semiconductor material is proposed (see Non-patent Literature No. 1). Diamond semiconductor has a deep impurity level, for example, when boron (i.e., B) is used as a P type impurity, the impurity level of diamond semiconductor is 0.37 eV, and, when phosphorus (i.e., P) is used as a N type impurity, the impurity level is 0.57 eV. Further, thermal voltage at room temperature is 0.026 eV. Thus, as shown in FIG. 7, a carrier density at room temperature is small. When the impurity density is not so large (e.g., 1×1018 cm−3 in the drawing), the diamond semiconductor provides band conduction, and therefore, a resistance of a semiconductor layer becomes very large. On the other hand, when the impurity density becomes larger so that a hopping conduction is dominant (e.g., about 1×1019 cm−3 or more), the resistance of the semiconductor layer is rapidly reduced. Here, in FIG. 7, T represents temperature indicated by kelvin (i.e., K). The value of 1000/T at room temperature is about 3.3.
In a semiconductor device having a PN junction, when a reverse voltage is applied to the PN junction between the P type layer and the N type layer, each of which has a high impurity density, The maximum electric field intensity at the PN junction becomes larger. Accordingly, a blocking voltage becomes smaller. When the impurity density s reduced in order to increase the blocking voltage, the maximum electric field intensity is also reduced, and the resistance of each of the P type layer and the N type layer becomes larger.
Accordingly, in the semiconductor device having the deep impurity level, when the blocking voltage is set to be larger, the resistance becomes larger. For example, in a FET having the large blocking voltage, the resistance is large, and therefore, a difficulty arises such that the conduction loss becomes larger.